The invention relates to a wire element for mixing in a subsequently hardening soft material. Such wire elements with a high modulus of elasticity and high tensile strength are used in great quantities for the reinforcement of synthetic resins, plastics, rubber and other materials, especially mortar and concrete, which only offer a small resistance to a tensile force. Those elements are short pieces of wire, made of steel, glass fiber or another similar material of high tensile strength which, when mixed up in a material with lower modulus of elasticity, results in a two phase material of remarkable strength.
In order to improve the strength characteristics for a given weight of wire material, it has been proposed to increase the concentration of wires in the mass by a greater subdivision of finer wires. A sufficient length to diameter ratio is necessary in order to assure a sufficient grip of the wire on the material. Several other wire forms have also been proposed in order to improve this grip, such as corrugations in the wire, helicoidal or ring shaped wires.
It has been observed, however, that the presence of wire elements in the material has a detrimental influence on the mixability. The elements have a tendency to knit together into balls, and the distribution of the wires is then insufficient. For straight wires it is known that this tendency increases with the concentration of wires in the mass and with the length to diameter ration. And these are exactly the conditions for good strength characteristics. From this, it may reasonably be deducted that, for a given shape of wire, there is an incompatibility between the conditions of good strength and good mixability.
The invention is based on the understanding that there must exist wire forms for which this incompatibility is less severe than for straight wires.
It is therefor an object of the present invention to provide a type of wire elements for which this incompatibility is greatly attenuated, and for which it is possible to obtain for a given strength, a better mixability, with the same percentage of steel.
According to the invention, the wire element is characterized by the fact that it consists of a substantially straight wire end, having at both extremities a bending in a form for which the hooking-in of two neighbouring elements is not possible.
A better understanding of the reasons which are assumed to be the cause of these advantages will be given here with the help of an explanation of what is assumed to happen, on one hand, when the reinforced material resists to crack propagation under a loading stress, and, on the other hand, when the elements are mixed in the soft material which will harden afterwards.
Under a loading stress, the propagation of any incipient crack is arrested by the wire element which the crack edge finds on its way. There, the wire takes over the concentrated local tensile force provoking the propagation of the crack, and this force is transmitted through the wire and redistributed over the surrounding matrix material via the contact surface where reaction forces are built up which resist any relative motion of the wire within the material. This is called further the grip of the wire in the material. For example, a straight wire element in the way of a perpendicularly incident crack takes over a pure tensile force, and the reaction forces are the also parallel with the wire axis and delivered by the adherence of the wire to the material. In the regions of curvature of a non-straight wire under tension, there are in addition, reaction forces which are perpendicular to the contact surface, and these add to the grip. So the grip depends on the length and the form of the wire.
To obtain a good mixability, it is desired to use wire elements which have a small length to diameter ratio. But when a wire element of a certain diameter is too short, then it cannot provide a comparable grip with respect to the tensile force which the wire is able to support, and it is then unnecessary and uneconomical to use them such a great diameter. The steel is used uneconomically and for a given percentage of reinforcement the strength leaves much to be desired. Consequently, for straight wires, when using shorter and shorter wire elements, the mixability improves, but it is impossible to go under a certain limit of length to diameter ratio without losing too much strength.
It is now possible, by using incurved elements such like helicoids or corrugated wires, to improve the grip of the element in the material, so that a lower length to diameter ratio limit is obtained and, for a same strength, shorter wire elements can be used, which have a better mixability. But here the incurvations may in turn have a detrimental effect on the mixability, in such a way that the total result would be negative. And here arises a first problem: to find a type of incurvations which permit to give a maximum increase of the grip, and so permitting to use shorter wires with better mixability, but which give a minimum of deterioration of the mixability directly due to the incurvations themselves.
There is a second problem in connection with incurvated wires: their effective length is smaller than that of a straight wire of the same length. By this is meant that a straight wire covers a greater area where the propagation of the crack edge can be stopped. A propagating edge has more chances to find on its way a straight wire, then a corrugated wire of the same wire length, and this is detrimental to the final strength of the material. Consequently the problem is to find a type of incurvations which, while giving a maximum increase of the grip, give a minimum loss of the effective length.
It can indeed be observed that for the wire elements according to the invention, a good effective length is obtained: the wire element is a substantially straight element. This means that the substantially straight central portion of the wire is at least as long as the part which forms the bendings at both extremities. A mathematical straightness is not necessary for the substantially straight central portion, and even not possible in practice. For calling this portion a substantially straight portion, it is sufficient that the integral over this straight portion, of the absolute value of the incurvation per unit of length or curvature is lesser than 45.degree..
While maintaining a maximum effective length, a good grappling of the elements in the material is obtained by the bendings on both extremities. By such an extremity having a bending is meant that the part of the wire on each side of the substantially straight portion has a length of at least 5 diameters and that the integral over this length of the absolute value of the incurvation per unit of length is at least 60.degree.. A wire extremity which does not satisfy those conditions can hardly be considered as a grappling extremity.
Finally it has been observed that the mechanism of balling up during the mixing operation depends on the type of wire element. Long straight wires are flexible and able to interwine. Short wire ends have more the tendency to enter with one extremity and to remain pinned in the forming ball and subsequently to pleat during further mixing and become a part of the ball. For substantially straight short wire elements with bendings on both ends this mechanism of pinning in does not seem to be possible, and nevertheless also a balling up phenomenon exists. It has been observed by the inventor that this happened especially with bendings in the form of a hook, and further observation showed that the balling up is due to the wire extremities which hook together. From this it may be concluded that the bending at the extremities of a straight wire, in a form where the hooking-in of two neighbouring elements is not possible, not only prevent the hooking-in mechanism, but also the pinning-in mechanism. Consequently a type of incurvation of a wire element has been found, which not only permits to give an increase in the grip, permitting to use shorter wires with better mixability, but which also procures a further improvement of this mixability. But it has also been explained that this type of incurvation was a type which, while giving a good grappling in the material, gives a minimum loss of effective length. And so the two problems relating to a good type of incurvation of a wire element have been solved. And it is believed that this is the explanation of the compatibility between strength of the material and mixability for the wire elements according to the invention. So it results that the length to diameter ratio, for which the best results are obtained is between 50 to 1 and 200 to 1.